Generally, a roll blind is a device mounted on a transparent glass window or glass wall for blocking light from the outside to control interior atmosphere or for covering the inside not to be seen from outside.
The conventional roll blind has a structure that one roll screen having the same light transmittance in whole area is wound around a bar for winding to block the light. Such a roll blind has a structure that a roll screen is wound up around a bar for winding fixed on upper portion and controlling the height of the roll screen is only a way to control the light transmission amount.
According to the way controlling the light transmission amount by controlling the height of roll screen, the light completely penetrates a part (the lower portion) of the glass window or glass wall and is completely blocked in a part (the upper portion), so it is impossible to uniformly control the light transmission amount in the whole area of glass window or glass wall. Therefore, required is a structure capable to uniformly control the light transmission amount in whole area.
To solve such a problem, a method for controlling the light transmission amount by-disposing two sheets of roll screens comprising light transmitting portions and light shielding portions alternately disposed widthwise or lengthwise to be overlapped and controlling the overlapped amount was proposed in the past.
However, most of such devices place emphasis on the function for controlling the light transmission amount and such devices become complicated and shoddy. That is, since such devices have a function controlling the light transmission amount, however, those are too big or complicated, some problems that the practicality and durability deteriorate are encountered.
Especially, since an operating apparatus for controlling the overlapped amount of two roll screens is needed separately from a ball-chain for operating a winding bar to control the light transmission amount, the operation of roll blind becomes complicated and the device become complicated. Accordingly, the cost increases and the durability and the credibility of the device deteriorate and such products are avoided in the market.
Accordingly, a roll blind having a structure enabling ascent and descent of roll screen and control of the overlapped amount of two sheets of roll screens with one ball-chain, a simple mechanical structure, and a good credibility in spite of repeated operations is required.
In addition, since there is a problem that the light shielding portions and the light transmitting portions of two sheets of roll screens are out of match in initial fabrication or by the deformation due to a long-time use, a structure that the light shielding portions and the light transmission portions of the roll screens can easily be reset is required.